1. Field of the Invention
The present invention relates generally to image pickup apparatus with a solid-state image pickup device, and is directed to an improvement in an image pickup apparatus which employs a solid-state image pickup device having a light receiving and charge transfer portion which comprises a plurality of light receiving areas, charge transfer areas, charge-reading gate areas for reading charge stored in the light receiving areas to the charge transfer areas, an output portion and so on, and is operative to obtain, at the output portion of the light receiving and charge transfer area, an image pickup signal output in response to charge produced and stored in the light receiving areas constituting the light receiving and charge transfer portion.
2. Description of the Prior Art
In a solid-state image pickup device used for a video camera producing video signals and the like, a light receiving and charge transfer portion comprising a plurality of light receiving areas arranged in horizontal and vertical rows for performing photoelectric conversion and charge transfer areas each composed of charge coupled devices (CCDs) or the like for transferring charge produced by light-reception to be stored in the light receiving areas, both of which are formed on a common semiconductor substrate, is provided. Such a solid-state image pickup device that has the light receiving and charge transfer portion can be employed in not only an image pickup device in which image pickup signal outputs representing moving pictures are obtained but also an image pickup device in which image pickup signal outputs representing still pictures each substantially stationary in an extremely short period are obtained.
When the solid-state image pickup device is employed in the image pickup apparatus for producing image pickup signal outputs representing still pictures, it is required to cause the light receiving areas constituting the light receiving and charge transfer portion to receive light from a subject in a very short period. Accordingly, for the solid-state image pickup device used for obtaining the image pickup signal outputs representing still pictures, a mechanical shutter is often placed in front of the light receiving and charge transfer portion to be made open and closed selectively for controlling arrival of light from the outside at the light receiving areas.
The mechanical shutter placed in front of the light receiving and charge transfer portion is normally closed to shut the light from the outside off so as not to reach the light receiving areas and made open to allow the light from the subject to arrive at the light receiving areas only during a predetermined short period. With such an operation of the mechanical shutter, each of the light receiving areas receives the light from the subject during the short period and stores therein charge produced in response to the still picture of the subject.
The charge stored in each of the light receiving areas is read through a charge-reading gate area formed in the light receiving and charge transfer portion to the charge transfer area and then transferred through the charge transfer area to an output portion of the light receiving and charge transfer portion as signal charge. In the output portion, the signal charges transferred thereto are converted into the image pickup signal output representing the still picture of the subject. The transfer of the charge in the charge transfer area is carried out with a transfer driving signal based on line and frame synchronous signals at every line period included in each frame period. Then, a video signal having line and frame periods and representing still pictures is produced on the basis of the image pickup signal output obtained from the output portion of the light receiving and charge transfer portion.
However, the above-mentioned image pickup apparatus for still pictures, in which the mechanical shutter is placed in front of the light receiving and charge transfer portion of the solid-state image pickup device, is accompanied with such problems that the mechanical shutter is complicated in structure to be considerably expensive, gradually deteriorated in mechanical accuracy with use over a long term to have lack of operation reliability, and an obstacle to miniaturization of the apparatus. Therefore, there has been also proposed an image pickup apparatus in which image pickup signal outputs representing still pictures can be obtained from a solid-state image pickup device employed therein without any mechanical shutter.
In such an image pickup apparatus for still pictures as not to have the mechanical shutter, a so-called electronic shutter is employed. According to the concept of the electronic shutter, charge which is produced by photoelectric conversion in each of light receiving areas constituting a light receiving and charge transfer portion of the solid-state image pickup device, to which light from the outside is normally introduced, is stored in each of the light receiving areas only during a short period determined by a shutter operation so as to be used for producing an image pickup signal output, and drained away to the outside of the light receiving areas and charge transfer areas without being stored during periods other than the short period determined by the shutter operation so as not to contribute to production of the image pickup signal output. The period determined by the shutter operation, during which the charge produced by photoelectric conversion in each of the light receiving areas is stored in that light receiving area, is called a light receiving period under the employment of the electronic shutter.
In the previously proposed image pickup apparatus for still pictures which employs the electronic shutter, the light receiving period is determined in such a manner as shown in FIGS. 1A to 1D. FIGS. 1A and 1B show respectively a frame synchronous signal SF and a line synchronous signal SH both taking part in the transfer of charge carried out by the charge transfer areas in the solid-state image pick up device. Further, as shown in FIG. 1C, a timing signal SX in synchronism with the frame synchronous signal SF is produced in the image pickup apparatus.
Under such a condition, when the shutter operation is set at a time point t0 shown in FIGS. 1A to 1D for causing the image pickup apparatus to begin an image pickup operation, a charge sweeping operation for sweeping away the charge in the light receiving areas and the charge transfer areas constituting the light receiving and charge transfer portion of the solid-state image pickup device to the outside of the light receiving and charge transfer portion, is started at a time point t1 corresponding to a front edge of the timing signal SX appearing first after the time point t0. This charge sweeping operation continues during a period predetermined for each solid-state image pickup device to correspond to, for example, 20 to 30 line periods. Then, as shown in FIG. 1D, a light-reception start signal STR having its front edge at a time point t2 is supplied to the solid-state image pickup device so that the light receiving period for the light receiving and charge transfer portion begins at the time point t2. The light receiving period for the light receiving and charge transfer portion terminates at a time point t3 corresponding to a front edge of the timing signal SX appearing first after the time point t2.
In this case, the light receiving period is so determined as to be the period from the time point t2 corresponding to the front edge of the light-reception start signal STR to the time point t3 corresponding to the front edge of the timing signal SX appearing first after the time point t2. The time point t3 corresponding to the front edge of the timing signal SX which determines the end of the light receiving period is fixed at a time point corresponding to a rear edge of the frame synchronous signal SF because the timing signal SX is in synchronism with the frame synchronous signal SF. Therefore, under the condition in which the light receiving period has a predetermined length of time, the time point t2 corresponding to the front edge of the light-reception start signal STR is so determined as to make the light receiving period having the predetermined length of time just before the time point t3 corresponding to the rear edge of the frame synchronous signal SF.
In the image pickup apparatus employing the solid-state image pickup device in which the light receiving period for the light receiving and charge transfer portion is determined in such a manner as mentioned above, when the shutter operation is set, the light receiving period begins in the period between the timing signals SX appearing respectively first and second after the time point at which the shutter operation is set and terminates at the time point of the timing signal SX appearing first after the time point at which the light receiving period begins. Since the timing signal SX is in synchronism with the frame synchronous signal SF, the light receiving period for the light receiving and charge transfer portion does not begin immediately after the shutter operation is set but begins when the timing signal SX appears first after the time point at which the shutter operation is set at the earliest or when a period having the length of time more than one frame period (for example, 1/30 seconds) has passed after the time point at which the shutter operation is set at more belated.
This means that a light-receiving response operation in which the period for waiting for the appearance of the timing signal SX and the succeeding period in which the charge sweeping operation is carried out pass away necessarily before the light receiving period for the light receiving and charge transfer portion begins, is performed in the image pickup apparatus. Accordingly, when the image pickup apparatus is actually used, a quick light-receiving response operation can not be obtained in the light receiving and charge transfer portion of the solid-state image pickup device and it is feared that the best opportunity for picking up a desirable momentary image of a subject is easily slipped. In other words, it is feared that a use of the image pickup apparatus misses often the best shutter chance.